Backup

In the field of information technology, backup refers to the copying of data so that these additional copies may be restored after a data loss event. more...

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Backups are useful primarily for two purposes: to restore a computer to an operational state following a disaster (called disaster recovery) and to restore small numbers of files after they have been accidentally deleted or corrupted. Backups differ from archives in the sense that archives are the primary copy of data and backups are a secondary copy of data. Backup systems differ from fault-tolerant systems in the sense that backup systems assume that a fault will cause a data loss event and fault-tolerant systems assume a fault will not. Backups are typically that last line of defense against data loss, and consequently the least granular and the least convenient to use.

Since a backup system contains at least one copy of all data worth saving, the data storage requirements are considerable. Organizing this storage space and managing the backup process is a complicated undertaking.

Storage, the base of a backup system

Data repository models

Any backup strategy starts with a concept of a data repository. The backup data needs to be stored somehow and probably should be organized to a degree. It can be as simple as a sheet of paper with a list of all backup tapes and the dates they were written or a more sophisticated setup with a computerized index, catalog, or relational database. Different repository models have different advantages. This is closely related to choosing a backup rotation scheme.

Unstructured 

An unstructured repository may simply be a stack of floppy disks or CD-R media with minimal information about what was backed up and when. This is the easiest to implement, but probably the least likely to achieve a high level of recoverability.

Full + Incrementals 

A Full + Incremental repository aims to make storing several copies of the source data more feasible. At first, a full backup (of all files) is taken. After that an incremental backup (of only the files that have changed since the previous backup) can be taken. Restoring whole systems to a certain point in time would require locating the full backup taken previous to that time and all the incremental backups taken between that full backup and the particular point in time to which the system is supposed to be restored. This model offers a high level of security that something can be restored and can be used with removeable media such as tapes and optical disks. The downside is dealing with a long series of incrementals and the high storage requirements.

Mirror + Reverse Incrementals 

A Mirror + Reverse Incrementals repository is similar to a Full + Incrementals repository. The difference is instead of an aging full backup followed by a series of incrementals, this model offers a mirror that reflects the system state as of the last backup and a history of reverse incrementals. One benefit of this is it only requires an initial full backup. Each incremental backup is immediately applied to the mirror and the files they replace are moved to a reverse incremental. This model is not suited to use removable media since every backup must be done in comparison to the mirror.

Continuous data protection 

This model takes it a step further and instead of scheduling periodic backups, the system immediately logs every change on the host system.

Read more at Wikipedia.org